Focus: Atomic Reflections

Cold atoms can be reflected from a solid surface in the same way that light reflects from a mirror.

Atomic beams can act like light waves and exhibit all of the classic wave effects, like interference and refraction. But atoms are usually guided with electromagnetic fields, not the solid materials that make lenses and mirrors. Now a Japanese physicist has demonstrated in the 5 February PRL that very cold atoms can reflect off of a solid surface the way light reflects from a mirror, rather than scattering in all directions. The results provide a new confirmation for the theory of so-called quantum reflection and a rare probe of the long-distance Casimir interaction between the atoms and the surface. The work could also pave the way for simpler atom manipulation equipment engineered from solids.

All waves, including light, reflect from an interface where the wave speed changes: Reflections dance on the surface of a swimming pool viewed from above or from below the surface because light moves more slowly through water than it does through air. A reflection occurs whether the light is moving from a faster toward a slower medium or from a slower toward a faster one.

Atoms in a beam feel a slight pull from the so-called van der Waals force when they get within a few micrometers of a solid surface. The attraction speeds them up and creates a reflective interface for atom-waves just above the surface, but only if the atoms aren’t moving so fast that they all go barreling through and hit the solid. In quantum terms, their quantum mechanical wavelength must be long enough that the change in wave speed is sudden–like hitting a wall or dropping off a cliff–rather than a gradual acceleration.

Quantum reflection was predicted in the 1930s but not observed until the 1990s, when researchers reflected hydrogen and helium atoms from a liquid helium surface. To observe reflection from a solid surface, Fujio Shimizu of the University of Electro-Communications in Tokyo needed atoms moving as slow as a few millimeters per second. He cooled neon atoms to 10 mK in a cloud less than 100 µm across, and then dropped them 39 cm to a clean silicon or glass plate. With the plate tilted less than one degree from vertical, the atoms bounced off at a shallow angle, and their velocity component in the horizontal direction was in the range of 1 to 30 mm/s, depending on the plate’s angle. Using a high resolution detector 73 cm below the plate, Shimizu picked up a small spot where the reflected atoms hit, close to the spot made by unreflected atoms (with the plate removed). The experiments accurately pinpointed the beam’s location by combining long atom path lengths with a good detector.

Shimizu plotted the “reflectivity” of the surface vs. the perpendicular velocity component and found rough agreement with the theory for atom-surface interactions. The data account for the standard van der Waals attraction, as well as the Casimir force, caused by the time delay for electromagnetic signals to travel between the surface and the atom when the atom is beyond a critical distance. That distance varies from a few hundred nanometers to a few micrometers, depending on atom velocity.

“Shimizu has demonstrated a new type of atomic mirror,” says Daniel Kleppner of MIT, and “he has done a beautiful job of measuring reflectivity.” The surfaces reflected about 30% of the atoms at best, so Shimizu prefers to say that a real atomic mirror is yet to come. He also hopes to develop other “atom optics” equipment using solid surfaces. Carlo Carraro of the University of California at Berkeley says that “performing such experiments is quite a tour de force” because of the low atomic velocities needed. Carraro points out that relatively few experiments can probe the feeble Casimir force, and Shimizu has helped to confirm the Casimir theory for this system.